This proposal seeks to transform our understanding of the causes and mechanisms of Alzheimer's disease. Alzheimer's disease is one of the most serious and economically important diseases for which there is no disease modifying therapy. The problem for therapeutic development in AD is that there is no commonly understood mechanism for AD pathogenesis. Human genetics implicates the APP and its proteolytic processing and the production of the amyloid A? peptide as a mediator of pathogenesis, but human clinical trials targeting the secretion of A? or the removal of plaques have demonstrated little or no effectiveness or have actually accelerated cognitive decline. Recent trials of gamma secretase inhibitors that prevent the secretion of A? reported that treated patients were cognitively worse than controls, suggesting that out understanding of the disease mechanisms is flawed or incomplete. The goal of this proposal is to identify and characterize the proteins and protein fragments that accumulate in and around the nuclei that are reactive to the fibril specific monoclonal antibody M78. We will test the hypothesis that APP and its amyloidogenic fragments misfold and aggregate into insoluble species prior to their proteolytic conversion to A? and accumulate intracellularly due to the intrinsic resistance of the aggregated A? domain to proteolysis. In preliminary studies we have discovered a novel amyloid fibril immunoreactivity in and around nuclei using a fibril specific monoclonal antibody, M78, which recognizes a discontinuous A? epitope in A? fibrils. Our studies suggest that M78 immunoreactivity colocalizes with both intracellular perinuclear APP and early extracellular plaque stages of AD prior to significant cognitive dysfunction and at intermediate times of pathogenesis in 3xTg-Ad mice. Some of the nuclear M78 immunoreactivity colocalizes with APP and A? as visualized with APP and A? specific antibodies suggesting that M78 immunoreactivity may represent misfolded, aggregated APP or APP fragments accumulating as intracellular amyloid. In 3xTg-AD mice, M78 positive plaques accumulate at later times that are not stained with 6E10 or 4G8, suggesting that a unique type of plaque is also identified by this monoclonal antibody. We also observed that the same spatial distribution of M78 and APP immunoreactivity in neuritic plaques is colocalized with DNA visualized with the fluorescent dye DAPI located in the central core of the neuritic plaque surrounded by a halo of APP immunoreactivity in dystrophic neurites. Together, our observations suggest a previously unidentified nuclear related intracellular pathway for amyloid pathogenesis and plaque biogenesis in AD. If this novel pathogenesis is a key event in AD, it will dramatically change our way of thinking about this disease, refocus the resources for therapeutic development and enable early clinical trials to alter the course of the disease.